(1) Field of the Invention
This invention relates to the casting of metals, particularly aluminum and aluminum alloys, by direct chill (DC) casting techniques. More particularly, the invention relates to the co-casting of metal layers by direct chill casting involving sequential solidification.
(2) Description of the Related Art
Metal ingots are commonly produced by direct chill casting of molten metals. This involves pouring a molten metal into a mold having cooled walls, an open upper end and (after start-up) an open lower end. The metal emerges from the lower end of the mold as a metal ingot that descends as the casting operation proceeds. In other cases, the casting takes place horizontally, but the procedure is essentially the same. Such casting techniques are particularly suited for the casting of aluminum and aluminum alloys, but may be employed for other metals too.
Casting techniques of this kind are discussed extensively in U.S. Pat. No. 6,260,602 to Wagstaff, which relates exclusively to the casting of monolithic ingots, i.e. ingots made of the same metal throughout and cast as a single layer. Apparatus and methods for casting layered structures by sequential solidification techniques are disclosed in U.S. Patent Publication No. 2005/0011630 A1 to Anderson et al. Sequential solidification involves the casting of a first layer (e.g. a layer intended as an inner layer or core) and then, subsequently but in the same casting operation, casting one or more layers of other metals on the first layer once it has achieved a suitable degree of solidification.
While these techniques are effective and successful, difficulties may be encountered when attempting to employ the sequential solidification technique with one or more alloys that have high coefficients of contraction upon solidification and cooling. In particular, when such a metal is employed as an inner layer forming a substrate for an outer layer of another metal, it is found that the inner layer may have a tendency to shear off the outer layer (or exhibit weakened adhesion) during the casting operation, especially at the extreme ends of a rectangular ingot cast with a layered structure, and especially during the initial stage of ingot formation.
It is known that the addition of other elements to pure aluminum changes its coefficient of contraction to a greater or lesser degree. Some elements increase the coefficient of contraction, while others reduce it. Elements such as magnesium and zinc increase the coefficient compared to pure aluminum, whereas elements such as copper, iron, silicon and nickel reduce the coefficient. The degree to which the coefficient is changed generally varies in an approximately linear manner with the percentage of the element added to the aluminum.
The difficulties referred to above, while potentially experienced with all sequentially-cast metal structures, tend to be more acute when an inner layer is made from an aluminum alloy that has a high coefficient of contraction and, especially, a higher coefficient than aluminum itself, particularly an aluminum alloy containing magnesium and/or zinc, especially when such elements are contained in relatively high concentrations, e.g. Mg in amounts more than about 2.5 wt. %. However, similar problems may be encountered when the coefficient of contraction of a metal of one layer is not particularly high, but there is a large difference between the coefficients of two adjacent layers, e.g. an alloy containing significant quantities of nickel in one layer and an alloy containing copper in an adjacent layer. While both these elements cause a reduction of the coefficient compared to pure aluminum, nickel has a much more negative effect on the coefficient than copper so that, depending on the relative concentrations of these elements, the difference in the respective coefficients can be quite large.
There is therefore a need for improved casting equipment and techniques when co-casting metals of these kinds.